Conventionally, in construction machines and the like, an axial hydraulic piston pump driven by an engine and an axial hydraulic piston motor driven by a high-pressure hydraulic oil have been widely used.
For example, the axial hydraulic piston pump includes a cylinder block, a plurality of pistons, and a valve plate. In the cylinder block, a plurality of cylinders are provided so as to rotate together with a rotational shaft rotatably provided in a case, extending in the axial direction, and separated from each other in the circumferential direction. The pistons are slidably inserted into the respective cylinders of the cylinder block and move in the axial direction along with the rotation of this cylinder block to suck and discharge the hydraulic oil. The valve plate is provided between the case and an end surface of the cylinder block. A suction port and a discharge port communicating with the respective cylinders are formed on the valve plate. In the hydraulic pump, when a driving shaft is driven and rotated, the cylinder block rotates together with an operating shaft in the case, and the pistons reciprocate in the respective cylinders of the cylinder block. The hydraulic oil sucked into the cylinders from the suction port is pressurized by the pistons and is discharged from the discharge port as high-pressure hydraulic oil.
Herein, a suction process is conducted in which, when a cylinder port of each cylinder communicates with the suction port of the valve plate, the pistons move in the direction in which the pistons protrude from the cylinders from the start point to the end point of the suction port to suck the hydraulic oil into the cylinders from the suction port. On the other hand, a discharge process is conducted in which, when the cylinder port of each cylinder communicates with the discharge port, the pistons move in a direction in which the pistons enter the cylinders from the start point to the end point of the discharge port to discharge the hydraulic oil in the cylinders into the discharge port. By rotating the cylinder block so as to repeat the suction process and the discharge process, the hydraulic oil sucked from the suction port into the cylinder at the suction process is configured to be pressurized at the discharge process and discharged to the discharge port.